Design considerations for solar power plant

As the demand for solar electric systems grows, progressive builders are adding solar photovoltaics (PV) as an option for their customers. This overview of solar photovoltaic systems will give the builder a basic understanding of: • Evaluating a building site for its solar potential • Common grid-connected PV system configurations and components • Considerations in selecting components • Considerations in design and installation of a PV system • Typical costs and the labor required to install a PV system • Building and electric code requirements • Where to find more information Emphasis will be placed on information that will be useful in including a grid-connected PV system in a bid for a residential or small commercial building. We will also cover those details of the technology and installation that may be helpful in selecting subcontractors to perform the work, working with a designer, and directing work as it proceeds. Evaluating a Building Site – While the Pacific Northwest may have good to excellent solar potential, not every building site will be suitable for a solar installation. The first step in the design of a photovoltaic system is determining if the site you are considering has good solar potential. Some questions you should ask are: • Is the installation site free from shading by nearby trees, buildings or other obstructions? • Can the PV system be oriented for good performance? • Does the roof or property have enough area to accommodate the solar array? • If the array will be roof-mounted, what kind of roof is it and what is its condition? Mounting Location – Solar modules are usually mounted on roofs. If roof area is not available, PV modules can be pole-mounted, ground-mounted, wall-mounted or installed as part of a shade structure (refer to the section “System Components/Array Mounting Racks” below). Shading – Photovoltaic arrays are adversely affected by shading. A well-designed PV system needs clear and unobstructed access to the sun’s rays from about 9 a.m. to 3 p.m., throughout the year. Even small shadows, such as the shadow of a single branch of a leafless tree can significantly reduce the power output of a solar module.1 Shading from the building itself – due to vents, attic fans, skylights, gables or overhangs – must also be avoided. Keep in mind that an area may be unshaded during one part of the day, but shaded at another part of the day. Also, a site that is unshaded in the summer may be shaded in the winter due to longer winter shadows.2 Orientation – In northern latitudes, by conventional wisdom PV modules are ideally oriented towards true south.3 But the tilt or orientation of a roof does not need to be perfect because solar modules produce 95 percent of their full power when within 20 degrees of the sun’s direction. Roofs that face east or west may also be acceptable. As an example, a due west facing rooftop solar PV system, tilted at 20 degrees in Salem, Oregon, will produce about 88 percent as much power as one pointing true south at the same location. Flat roofs work well because the PV modules can be mounted on frames and tilted up toward true south. Tilt – Generally the optimum tilt of a PV array in the Pacific Northwest equals the geographic latitude minus about 15 degrees to achieve yearly maximum output of power. An increased tilt favors power output in the winter and a decreased tilt favors output in the summer. In western Washington and Oregon, with their cloudier winters, the optimum angle is less than the optimum east of the Cascades. Required Area – Residential and small commercial systems require as little as 50 square feet for a small system up to as much as 1,000 square feet. As a general rule for the Pacific Northwest, every 1,000 watts of PV modules requires 100 square feet of collector area for modules using crystalline silicon (currently the most common PV cell type). Each 1,000 watts of PV modules can generate about 1,000 kilowatt-hours (kWh) per year in locations west of the Cascades and about 1,250 kWh per year east of the Cascades. When using less efficient modules, such as amorphous silicon or other thin-film types, the area will need to be approximately doubled. If your location limits the physical size of your system, you may want to install a system that uses more-efficient PV modules. Keep in mind that access space around the modules can add up to 20 percent to the required area. Roof Types – For roof-mounted systems, typically composition shingles are easiest to work with and slate and tile roofs are the most difficult. Nevertheless, it is possible to install PV modules on all roof types. If the roof will need replacing within 5 to 10 years, it should be replaced at the time the PV system is installed to avoid the cost of removing and reinstalling the PV system. Photovoltaic System Types Photovoltaic system types can be broadly classified by answers to the following questions: • Will it be connected to the utility’s transmission grid? • Will it produce alternating current (AC) or direct current (DC) electricity, or both? • Will it have battery back-up? • Will it have back-up by a diesel, gasoline or propane generator set? Here we will focus on systems that are connected to the utility transmission grid, variously referred to as utility-connected, grid-connected, grid-interconnected, grid-tied or grid-intertied systems. These systems generate the same quality of alternating current (AC) electricity as is provided by your utility. The energy generated by a grid-connected system is used first to power the AC electrical needs of the home or business. Any surplus power that is generated is fed or “pushed” onto the electric utility’s transmission grid. Any of the building’s power requirements that are not met by the PV system are powered by the transmission grid. In this way, the grid can be thought of as a virtual battery bank for the building. Common System Types – Most new PV systems being installed in the United States are grid-connected residential systems without battery back-up. Many grid-connected AC systems are also being installed in commercial or public facilities. The grid-connected systems we will be examining here are of two types, although others exist. These are: • Grid-connected AC system with no battery or generator back-up. • Grid-connected AC system with battery back-up. Is a Battery Bank Really Needed? – The simplest, most reliable, and least expensive configuration does not have battery back-up. Without batteries, a grid-connected PV system will shut down when a utility power outage occurs. Battery back-up maintains power to some or all of the electric equipment, such as lighting, refrigeration, or fans, even when a utility power outage occurs. A grid-connected system may also have generator back-up if the facility cannot tolerate power outages With battery back-up, power outages may not even be noticed. However, adding batteries to a system comes with several disadvantages that must be weighed against the advantage of power back-up. These disadvantages are: • Batteries consume energy during charging and discharging, reducing the efficiency and output of the PV system by about 10 percent for lead-acid batteries. • Batteries increase the complexity of the system. Both first cost and installation costs are increased. • Most lower cost batteries require maintenance. • Batteries will usually need to be replaced before other parts of the system and at considerable expense. System Components Pre-engineered photovoltaic systems can be purchased that come with all the components you will need, right down to the nuts and bolts. Any good dealer can size and specify systems for you, given a description of your site and needs. Nevertheless, familiarity with system components, the different types that are available, and criteria for making a selection is important. Basic components of grid-connected PV systems with and without batteries are: • Solar photovoltaic modules • Array mounting racks • Grounding equipment • Combiner box • Surge protection (often part of the combiner box) • Inverter • Meters – system meter and kilowatt-hour meter • Disconnects: - Array DC disconnect - Inverter DC disconnect - Inverter AC disconnect - Exterior AC disconnect If the system includes batteries, it will also require: • Battery bank with cabling and housing structure • Charge controller • Battery disconnect 1. Basic Principles to Follow When Designing a Quality PV System 1. Select a packaged system that meets the owner's needs. Customer criteria for a system may include reduction in monthly electricity bill, environmental benefits, desire for backup power, initial budget constraints, etc. Size and orient the PV array to provide the expected electrical power and energy. 2. Ensure the roof area or other installation site is capable of handling the desired system size. 3. Specify sunlight and weather resistant materials for all outdoor equipment. 4. Locate the array to minimize shading from foliage, vent pipes, and adjacent structures. 5. Design the system in compliance with all applicable building and electrical codes. 6. Design the system with a minimum of electrical losses due to wiring, fuses, switches, and inverters. 7. Properly house and manage the battery system, should batteries be required. 8. Ensure the design meets local utility interconnection requirements. 1.2. Basic Steps to Follow When Installing a PV System 1. Ensure the roof area or other installation site is capable of handling the desired system size. 2. If roof mounted, verify that the roof is capable of handling additional weight of PV system. Augment roof structure as necessary. 3. Properly seal any roof penetrations with roofing industry approved sealing methods. 4. Install equipment according to manufacturers specifications, using installation requirements and procedures from the manufacturers' specifications. 5. Properly ground the system parts to reduce the threat of shock hazards and induced surges. 6. Check for proper PV system operation by following the checkout procedures on the PV System Installation Checklist. 7. Ensure the design meets local utility interconnection requirements 8. Have final inspections completed by the Authority Having Jurisdiction (AHJ) and the utility (if required).